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Focus stacking allows you to create an image with a greater apparent depth of focus (i.e. more of the subject in focus) than would otherwise be possible.
This is used in fields like macro photography where you are often working at the limits of the capabilities of the lens.

If you are taking photos of something stationary, then a focusing rail will allow you to do them perfectly. If you are talking about moving things, like insects, then all you can do is take a lot of photos. If you aren't able to focus reliably without a tripod and rail, then you can use burst mode. But, with some practice, you can get to the point where ...

Always use as much data as you can. It's actually easier to reduce noise when there's more information to begin with. (Reducing resolution is a brute-force noise removal tool, throwing away both noise and signal.)
If you're still concerned about the size final image, reduce the resolution at that point, after flattening.

Use focus stacking.
Why would a photographer do focus stacking?
What are the best practices for DOF stacking?
What software is available for macro focus stacking?
What are the disadvantages of focus stacking?

IMO focus stacking isn't very useful for landscape photography, it's more useful in macro photography, but for landscapes there are better ways to get good focus. check out this online depth of field calculator to get an idea about DOF in different settings.
Anyways, about your question, I must say it's possible to get a good result in focus stacking if you ...

You are right about the disadvantages of focus stacking, also, the process of taking pictures for focus stacking (taking multiple pictures at different focus ring positions and getting every bit of the picture sharp in at least one picture) can be a bit technically challenging.
That is why traditionally landscape photographers solve this issue by using a ...

An alternative to focus stacking is to tilt the plane of focus so that it is parallel to the object you are trying to photograph. Usually the plane of sharpest focus is parallel to the film/sensor plane of the camera, however by tilting the lens relative to the camera the plane of focus can be tilted so that it is aligned with the longest axis of your ...

In Photoshop CS5 or later, load your images into layers.
Select Edit > Auto Align Layers (at macro distances even on a tripod small changes in focus will alter the perspective of each shot)
Select Edit > Auto Blend Layers. This will select the sharpest parts of each layer and create a mask. The masks can be quite complex, but it does a very good job.
...

A friend of mine recommended ImageJ in combination with the Stack Focuser plugin for combining the images into a single image with an extended depth of field. He mainly uses it for microscope images.
I've also found good references for CombineZP. Although there's not much info on the site there is a yahoo user group for it for more information and this ...

You can also use the hugin toolset which includes the align_image_stack and enfuse tools. You may also wish to add the EnfuseGUI to cut down on the typing.
Works very well
Free & Open Source
Multi-platform runs on Mac/Windows/Linux
There is a very nice walk through here.

As I know, depth of field is not exactly the same in front of and behind focus plane. One third of area in focus is in front of focused point and two thirds are behind this plane.
So if you want to have whole your object in focus, you should focus to some point which is approximately in one third of depth of object. Especially when you don't have closer ...

You are simply making the wrong assumptions. Just "playing around" and thinking you see a correlation doesn't mean that it actually exists. You've just fixed the sensor format at APS-C, aperture at f/22 and tried varying the focal length and subject distance a bit and thought that you saw a correlation.
However if you try these settings for example: f/2.8, ...

I don't have too much to add, but I have some limited experience with focus stacking on microscopes that I can share.
I've done focus stacking on an SEM, the following image is a stack of only two images. I wish I had a third to fill in the blur in the middle.
Image of a flower I took with a scanning electron microscope. The near foreground and the 'sky' ...

I have just written a focus-bracketing script for closeup lenses, including the Raynox 250.
Here is a link to my article, which contains documentation of the script as well as the script itself:
http://www.dpreview.com/articles/9780512447/focus-bracketing-scripts-for-macro-setting-and-for-closeup-lenses
Here is another link to some outdoor stacked images ...

I think it is worth mentioning that if you are a Canon person, Magic Lantern has a great focus stacking utility that will let you set the number of shots in front and the number behind as well as the size of the focus shift for each shot. It will take the series of shots and even generate a little script that can be used with Hugin to merge.